The invention concerns, in general, the manufacture of complex porous metallic or metallized structures.
This invention relates more particularly to the manufacture of complex porous metallic or metallized structures for application as electrodes for the electrolysis of liquid effluents, as electrode supports for electrochemical generators, as catalyst supports, filtration media, phonic insulation, electromagnetic and nuclear protection structures, or for other applications.
The metallic or metallized structures according to the invention are of the foam, felt or fabric type having a high level of open porosity, and having the aspect of a dense network of fibers or mesh with a three dimensional skeletal structure defining a plurality of open spaces intercommunicating with one another and with the exterior of the structures.
Foams are reticulated cellular structures of high porosity (greater than 80%, and possibly reaching 98%) and having an open porosity by inhibiting cell wall formation, wherein the totality of the network""s openings, or at least a high proportion thereof, are in communication with one another.
Felts are randomly interlaced matted fibers defining therebetween inter-fiber spaces of variable shapes and dimensions, communicating with one another.
Fabrics are structures constituted by an assembly of textile threads or fibers that are interlaced, either woven or netted. They may be in the form of thick and complex structures, in particular when they are made of two external woven faces connected by knitted threads that hold them simultaneously spaced apart and interconnected, as for example can be produced using Raschel type knitting machines.
These various complex porous structures, that according to the invention will be metallized throughout their entire thickness, over all their developed surface, without clogging of their porosity, may be provided starting from various base materials.
For foams, organic, mineral or synthetic materials are used, and in particular polymers such as polyester, polyamide, polyurethane or polyether.
For felts and fabrics, organic mineral or synthetic materials are also used such as the previously-cited polymers, or glass, rock or carbon fibers, or natural fibers such as cotton, wool or the like.
Various processes for metallizing such structures have already been proposed, including:
chemical deposition, followed by one or several electrochemical depositions,
deposition of carbon or graphite particles, followed by one or several electrochemical depositions,
vacuum deposition of metals, in particular by cathodic vaporization, gas diffusion or ionic deposition, followed by one or several electrochemical depositions,
chemical vapor deposition.
Whenever electrochemical deposition will be carried out, one should previously prime the surface to be electroplated, to render it electrically conductive. This is the purpose of the xe2x80x9cpre-metallizingxe2x80x9d stage incorporated in most of the cited processes (chemical deposition, deposition of carbon particles, vacuum deposition).
The present invention is concerned particularly with carrying out a pre-metallization process in the manufacture of complex porous metallic structures, which process provides various advantages relative to the previous techniques for the production of said products.
Chemical deposition on an industrial scale is an expensive process and is somewhat difficult to control. It involves the consumption of expensive chemical products (tin, palladium, . . . ) and necessitates, between each of its steps, careful rinsing operations on complex porous structures which have a high retentive power, in order to avoid undesirable contamination by transfer of the reactive components from one treatment bath to the next. This process generally provides a very efficient pre-metallization, in particular with a high penetrating power in the structures to be treated, but generates additional costs due to the necessity of retreating its liquid effluents.
The deposition of carbon or graphite particles, which is widely used up to date on an industrial scale for the production of metallic foams, is relatively inexpensive, both in terms of the products consumed and as regards the investments necessary for carrying it out. It however has three types of drawback:
the carbon particles do not form a continuous conductive deposit on the surface of the structure""s openings. Electrochemical metallization therefore has to provide a bridging of these particles between one another. In some cases, the initial phase of propagation of the electroplated deposit through the entire volume of the structure is slow, and should preferably be carried out with enhancement of the structure""s conductivity using metallic anodic contacts, in order to achieve economically acceptable recovery rates;
because of the size of the carbon particles, it is not possible with this method, without clogging the internal porosity, to treat denser structures: foams having a porosity greater than 100 ppl (100 pores per linear inch), dense felts, or thick and dense fabrics made of fine fibers whose external woven faces are connected by knitted threads that maintain the external faces simultaneously spaced apart and interconnected;
the deposition of carbon particles complicates the step of pyrolisis of the organic materials after metallization, due to a substantial increase of their mass.
Among vacuum deposition methods, only cathodic vapor deposition is industrially used for the pre-metallization of complex porous structures like those used in the invention. This method, described in French Patent 2.558.485 of Jan. 25, 1984, is generally regarded as the most efficient one for the application under consideration.
It however requires the use of sophisticated industrial devices, precise and delicate operating procedures must be followed, and the investment in production equipment is relatively high. At the present time, there are two limitations with this process:
although it enables a homogeneous continuous deposition within complex porous structures with a high penetrating power, the process nevertheless has limitations in terms of the thickness of the substrate to be treated and the density of fibers or openings (these two criteria being combinable), in particular when the substrate is constituted of an organic material that must not be subject to great increases of temperature;
at the present time, this method enables a batch operation (for the treatment of plates) or a semi-continuous operation (treatment using rollers) but not a truly continuous operation at an economically acceptable cost.
The pre-metallization processes summarized above are satisfactory from the point of view of industrial production on a large scale, but nevertheless have the described drawbacks.
It is furthermore generally speaking always desirable to reduce the cost of known processes and to simplify the carrying out of these processes, in order to reduce the cost of the resulting products. These are two of the advantages obtained by the present invention.
The present invention, through a specific adaptation of a process which is known for applications that are simpler to carry out, aims to permit the electroplating (galvanic metallization) of the above-defined porous complex structures by providing a pre-metallization by the preliminary deposition of a conductive polymer, so the later metallization can then itself be carried out in specific conditions related to the nature of the pre-metallization layer.
The use of conductive polymers has already been described to permit electroplating to be carried out. This use, as with other prior pre-metallization techniques, has been designed, defined and made operational for application to simple or relatively simple surfaces: smooth surfaces, or smooth surfaces connected by holes of diameter greater than 0.5 mm, the depth of the holes being of the order of 1 mm. The prior use notably concerned the pre-metallization of printed circuit boards.
The pre-metallization treatment of the complex porous structures as described above, by means of a deposit of conductive polymers, does not follow directly from the way the process has been used for treating simple surfaces. Carrying out this treatment, which underwent several unsuccessful trial-and-error attempts to develop it, as will be understood from the following explanations.
As is known, the deposition of metals or alloys by well known techniques that are mastered for smooth surfaces, may be impossible to carry out or may require the implementation of specific methods when it is desired to carry them out on complex surfaces. For example, this is so for chromium plating. The same applies to the production of a continuous deposit of a conductive polymer on the porous complex structures of this application, on industrial scale, and which enables the subsequent electrochemical deposition throughout all of the thickness of the structure, on all of its developed surface, without clogging its porosity.
The specifics of developing a suitable deposition technique come not only from the extreme tortuosity of the structures to be treated, their thickness, the low inter-fiber space or pore dimension, but also from the chemical nature of the base material (usually organic) and its configuration in a particularly divided form (fibers and openings of several microns thickness).
The conductive polymer layer is obtained by polymerization of a monomer deposited on the fibers or openings of the substrate to be treated. This polymerization is carried out by oxidation-doping of the monomer.
Certain monomers such as pyrrole, furane, thiophene and certain derivatives thereof, and in particular functional monomers, have the property of being oxidizable into polymers having the characteristic of being electrically conductive.
In some of the processes of this type, the polymer deposition takes place in the gas phase, but it will easily be understood that such a gas-phase process is complex and difficult to carry out within highly porous structures.
In other processes which concern the treatement of porous or non-porous surfaces (but which are poorly adapted for the specific treatment of very porous surfaces), all of the steps are carried out in the wet (liquid) phase in precise operating conditions which were said to provide the advantage of dispensing with the need for any rinsing between the different steps. However, this is clearly contrary to the teaching of the present invention, as will be explained in detail later. It is thought that this process, as well as the previously mentioned one, underestimates the inherent constraints of porous surfaces and hence claims to be equally applicable to all types of surface.
Finally, other processes carry out polymerization of a monomer deposited on the surface""s support by anodization, which radically differenciates them from the process according to the invention.
The invention precisely concerns complex porous structures of the reticulated foam, felt or fabric type, which are pre-metallized throughout their structures, and a process for manufacturing such structures comprising the following sequential steps:
(1) oxidizing pre-treatment of the base structure,
(2) rinsing, possibly followed by draining and drying,
(3) deposit in a wet phase, on the surface of the fibers or openings of the structure, of a monomer which in a polymerized form is electrically conductive,
(4) possible natural draining and/or forced draining,
(5) polymerization by oxidation-doping of the monomer into an electrically conductive polymer,
(6) rinsing, and possible draining,
(7) possible drying.
wherein these steps are carried out within the structure, throughout its entire thickness, over the surface of each of its fibers or openings, without clogging its pores, and steps (3) to (7) may be repeated in the same order several times.
The invention also concerns a process for producing complex metallic or metallized porous structures, comprising the steps of pre-metallization with a conductive polymer and electroplating of metals on organic or mineral foams, felts or fabrics, possibly followed by pyrolysis of the original materials and of the pre-metallization components, and a heat treatement under controlled atmosphere of the metallic deposit or deposits, in which the pre-metallization comprises the following steps:
(1) oxidizing pre-treatment of the base structure,
(2) rinsing, possibly followed by draining and drying,
(3) deposit in a wet phase, on the surface of the fibers or openings of the structure, of a monomer which in a polymerized form is electrically conductive,
(4) possible natural draining and/or forced draining,
(5) polymerization by oxidation-doping of the monomer into an electrically conductive polymer,
(6) rinsing, and possible draining,
(7) possible drying.
wherein these steps are carried out within the structure, throughout its entire thickness, over the surface of each of its fibers or openings, without clogging its pores, and steps (3) to (7) may be repeated in the same order several times.
Hence, contrary to what has been discussed above, the rinsing steps are important steps as regards the quality of the pre-metallization layers obtained and, consequently, of the final electrodes. The final rinsing is particularly significant because it enables impurities such as iron, which could remain in the finished product after the electroplating and thermal treatments, to be eliminated.
By xe2x80x9cwet phasexe2x80x9d is hereafter meant a treatment in the liquid phase, for example in a solution, or a treatment in contact with a mist which, by definition, contains fine droplets of liquid.
The initial oxidizing pre-treatment step (1) becomes critically important for successful pre-metallization when carried out within a complex porous structure. This is because the electroplating, which follows the pre-metallization treatment, must be carried out as regularly as possible, within the structure itself over the entire surface of each of said structure""s fibers or openings. The metallic structure must be the exact replica of the initial structure before treatment. It is therefore required that each fiber or opening, in the entire internal volume of the structure as well as on its surface, must be rendered conductive during the pre-metallization treatment. For this, it is indispensable that deposition of the monomer, before its later conversion into a conductive polymer, must have taken place over the entire surface of the fibers or openings, without clogging the surface and internal porosity. The monomer must be properly attached on the fibers or openings, and not in a more-or-less free manner within the structure with part filling or with bridging the inter-fiber spaces or the pores.
It has been observed that should the oxidizing pre-treatment prior to depositing the monomer not have been carried out in an appropriate manner, the monomer tends to precipitate in a disordered fashion within the structure without following the initial architecture thereof and does not coat all of the fibers or openings of these thick, dense and complex structures.
The oxidizing pre-treatment of the invention is preferably but not exclusively carried out by immersing the structure in a solution containing permanganate/manganate salts and/or cerium IV compounds. It may also be carried out by spraying said solution within the structure, or by holding the structure in a mist of the same composition.
The oxidizing pre-treatment has an essential double function in producing products according to the invention:
it localizes precipitation of the monomer solely on the surface of the fibers or openings within the complex porous structure,
it takes part in polymerizing the monomer from the lower or hidden face of deposit.
The pre-treatment, in order to prepare for a proper deposit of the monomer, provides a superficial etching of the fibers or openings constituting the skeleton of the structure to be treated. Surface priming is carried out by etching the constituent organic material, which produces a surface microporosity leading to the production of an excellent bonding surface for the later deposit of the monomer, which will then preferentially precipitate onto this bonding surface instead of in a disordered manner in the porosity of the structure. Clogging of the structure to be processed can thus be avoided.
Obviously, these considerations are not a cause for concern when the pre-metallization and metallization have to be made on simple or relatively simple surfaces, for example non-porous surfaces.
The second function of this oxidizing pre-treatment is equally fundamental when one considers that the metallization to be carried out following pre-metallization with the conductive polymer must itself be carried out within the complex porous structure, through its entire thickness, by a homogeneous and continuous coating of the fibers on openings, without clogging the internal and surface porosity. To correctly electroplate such dense three-dimensional structures through to the core of the structure requires that the pre-metallization must provide each point thereof with a sufficient electrical conductivity.
The fineness of the fibers or openings, the thinness of the monomer deposit and hence of the polymer obtained by oxidization thereof, and the limited intrinsic conductivity of the polymer obtained all necessitate a polymerization of the precipitated monomer which is as complete as possible.
For this, it is necessary that oxidation of the layer of monomer should undergo oxidizing attack which leads to its polymerization, not only from its outer surface, but also from its lower hidden face adhering to the bonding surface of the structure""s fibers or openings. This lower face is protected from the oxidizing-doping action of the above mentioned step (5). To obtain a sufficiently polymerized pre-metallization layer, and consequently a sufficiently conductive layer, it is very important that polymerization of the monomer may take place from both sides.
There is no such constraint when the subsequent electroplating has to be made on a simple or relatively simple surface. The operative conditions for the oxidizing pre-treatment are also considerably less critical when, in the known case of the treatment of printed circuit boards, the material to be treated is in the form of a continuous thick board, of the order of one millimeter thick. The etching produced by the oxidizing pre-treatment cannot rapidly and undesirably damage such a board being etched.
All this is quite different when treating complex porous structures according to the invention, whose fibers or openings have a thickness of the order of several microns. If the oxidizing etching treatment is not carried out in an appropriate way, it may weaken the structure or lead to rupture of the fibers or openings. The acidity of the pre-treatment solution, its temperature and the duration of the operation, as well as the efficiency of the subsequent rinsing step or steps constitute in this case critical parameters which have to be specifically adapted to the treated structure.
The second step (2) of the pre-metallization process is consequently rinsing the structure after the oxidizing treatment. The structures according to the invention, be they foams, felts or fabrics, have a high liquid retention capacity. It is thus important, in order that the oxidizing-etching step of the pre-treatment should not be undesirably protracted, to proceed, immediately after the end of said pre-treatment, with a careful rinsing phase. Depending on the treated material""s structure, this operation can be carried out in one or several rinsing steps, in a neutral or slightly acid medium, in a static bath, a flowing rinsing bath and/or with spraying of the structure. Such expedients are contrary to the teachings of the literature cited above. Draining operations may be carried out.
After each wet treatment stepxe2x80x94steps (1), (3) and (5) of the processxe2x80x94a draining of the structure before any optional rinsing steps that prove necessary enables excess fluids and active components to be economically recuperated. This may also be followed by drying.
The third step (3) of the pre-metallization process according to the invention consists in depositing or precipitating the selected monomer.
This step is preferably carried out by immersing the structure in a solution of the said monomer. It may also, within the scope of the present invention, be carried out by spraying said solution within the structure, or by holding the structure in a mist of the same composition.
One or several monomers whose polymerized form is electrically conductive can be deposited at choice, preferably, but not exclusively, selected from amongst pyrrole, furane, thiophene, and/or derivatives thereof.
It is important and is inherent in the invention to make a careful selection of the monomer and of all of the components of the precipitating solution. The double objective at this stage is:
to avoid the use of a monomer and of a solvent which, on an industrial scale, would be liable to lead to security problems (toxicity for the operators, a danger of pollution by generating liquid effluents to be retreated, formation of a polymer whose elimination after metallization would itself lead to dangerous gaseous effluents),
to employ products whose material cost and whose cost of implementation are compatible with the need to seek an economical pre-metallization process.
Finally to carry out the method according to the invention of precipitating the monomer in a complex porous structure, the monomer solution must have a viscosity compatible with a good penetration into the structure, in such a manner as to easily impregnate said structure throughout its entire thickness, without any obstruction and without formation of air bubbles within the internal pores, which would be detrimental to achieving a continuous precipitation over the entire surface of the fibers or openings of the structure.
Step (4) of the pre-metallization treatment of structures according to the invention consists in a possible natural draining and/or forced draining of the structures. This step will determine the quality of the finished products.
The fifth step (5) is that of polymerizing the monomer or monomers. This is preferably but not exclusively achieved in the invention by immersing the structure to be treated in an oxidizing-doping solution selected to lead to the desired polymer or polymers. It may also, within the scope of the present invention, be carried by spraying said solution within the structure, or by holding the structure in a mist of the same composition.
This polymerization takes place starting from the outside face of the monomer deposited in step (3), and progresses into the inside of the layer of monomer precipitated on the fibers or openings of the structure.
As has been seen above, it is important that polymerization should also develop through the inside face of the monomer deposit, by a reaction achieved by careful selection of the oxidizing pre-treatment of step (1), and which starts to occur as from step (3), precipitation of the monomer.
Polymerization must be followed by a step (6) of rinsing the structure, which may be carried out in the same manner as the rinsing of step (2), both of which operations are equally indispensable.
The pre-metallization treatment must be completed by a natural or forced drying step (7) of the treated structure.
Steps (3) to (7) may possibly be repeated in the same sequence to obtain the desired polymer deposit providing sufficient electrical conductivity for the subsequent metallization.
In order to provide a clear idea, the various criteria and parameters which should preferably be followed to obtain exploitable results are given in the following Examples and Table, which are given by way of non-limiting example.
We will hereafter describe a complete pre-metallization treatment according to a preferred embodiment of the invention by depositing a conductive polymer, followed by the electroplating of a complex porous structure.